The present invention relates to a two color electrostatographic apparatus such as a copying machine, printer for a facsimile system or the like.
A novel and unique two color electrostatic copying machine is disclosed in copending United States patent application Ser. No. 912,273, filed June 5, 1978, entitled "COLOR ELECTROSTATOGRAPHIC PROCESS AND MATERIAL FOR PRACTICING SAME": which is assigned to the same assignee as this application. The present invention constitutes improvements to the basic copying machine which yet further increase the copy quality.
Color electrostatic copying machines which produce full color copies are known in the art. These are generally of two types. The first type comprises a single photoconductive drum or belt which is exposed to a light image of an original document three times through filters of three primary colors respectively. After each imaging operation, a toner substance of a corresponding color is applied to the drum to form a color toner image which is transferred to a copy sheet. In this manner, three color toner images are sequentially formed on the drum and transferred to the copy sheet in register to produce a color copy. Often, a fourth black toner image is formed and transferred to the copy sheet in register with the three color toner images.
In such a copying machine it is essential that the toner images be transferred to the copy sheet is perfect register. The control mechanism for such a copying machine is therefore intricate and expensive. The three or four imaging operations for each copy require a disproportionate amount of time, making the process very slow.
The second type of color copying machine is much faster in operation but also much more expensive to manufacture. Such a copying machine comprises three or four photoconductive drums or belts. The original document is passed over all of the drums in one scanning movement, sequentially imaging the drums through three respective primary color filters. A toner development unit is associated with each drum. The copy sheet is fed through the machine in one pass, with the toner images being transferred thereto in register through sequential engagement with the drums.
In addition to the increased cost of the three or four drums compared to only one drum or belt in the first type of color copying machine, an intricate mechanism is also required in the second type of copying machine to ensure perfect register of the three of four toner images on the copy sheet.
A full color copying machine is unnecessary in many business operations where only commercial documents are copied, since such documents generally only comprise the colors black and red, in addition to a white background. This is because accounting records and the like generally contain credit entries in black and debit entries in red. Since in many such documents the debit and credit entries may be distinguished from each other only by the color of ink, many offices have purchased or leased full color copying machines for copying such records. The full color copying capability is wasted since it is only necessary to distinguish red from black on the copies.
The electrostatic copying machine disclosed in the above mentioned copending patent application utilizes only a single drum on which are formed at least two photoconductive layers having different spectral sensitivities. The drum is charged at least twice with different polarities and exposed to a light image of an original document bearing at least two colors such as black and red on a white background. The photoconductive layers conduct in different ways to form a bipolar electrostatic image. For example, red image areas may have a positive polarity, black image areas may have a negative polarity and white areas will have no charge. The electrostatic image is developed by means of negatively charged red toner particles which adhere to the positive red electrostatic image areas and positively charged black toner particles which adhere to the negative black image areas. The resulting two color toner image is transferred and fixed to a copy sheet to provide a finished reproduction of the original document.
The toner particles may be mixed together and applied to the drum in a single step. However, it has been found that better image quality is obtainable by applying the toners to the drum separately. For example, a first developing unit will apply the red toner to the drum and then a second developing unit will apply the black toner to the drum.
A problem has remained heretofore unsolved in that the toner image formed in the first developing step is degraded in the second developing step. In some cases the first toner comes off the drum in the second developing step, thereby producing a toner image of insufficient density. The first toner mixes with the second toner to contaminate the second toner. Where the second developing unit must be disposed above the first developing unit to accommodate the overall layout of the copying machine, the second toner has a tendency to drop down from an unavoidable small gap between the second developing unit and the drum into the first developing unit to contaminate the first toner. It will be understood that the two toners are charged to opposite polarities and are electrostatically attracted to each other. Combination of the two toners produces an electrostatically neutral substance which is incapable of producing a toner image. It has also been found that toner image degradation occuring in the second developing unit produces toner images with blurred edges and mixed colors.